1. Field of the Invention
The present invention relates to a method and a program for identifying geometric errors of a machine having translational and rotational drive axes.
2. Description of Related Art
As an example of a machine having translational and rotational drive axes, there is known a machine tool for processing parts or metal molds. Such a machine tool can process a workpiece into a desired shape by a material removal process including relative movements of both or either of a tool and a workpiece while rotating the tool or the workpiece.
As an example of this machine tool, FIG. 1 shows a schematic view of a three-axis control machining center (three-axis machine) having three translational axes. A spindle head 2 is allowed to make a translational movement with 2 degrees of freedom with respect to a bed 1 along an X-axis and a Z-axis, which are translational axes orthogonal with each other. A table 3 is allowed to make a translational movement with 1 degree of freedom with respect to the bed 1 along a Y-axis which is a translational axis orthogonal to the X-axis and the Z-axis. Accordingly, the spindle head 2 has 3 degrees of freedom of translational movement with respect to the table 3. The machine tool is driven using a servo motor (not shown) controlled by a numerical controlling device. A workpiece is secured to the table 3 and a tool is attached to the spindle head 2 and being driven to rotate. A relative position between the workpiece and the tool is controlled and a processing is performed.
As another example of this machine tool, FIG. 2 shows a schematic view of a five-axis control machining center (five-axis machine) having three translational axes and two rotational axes. A spindle head 2 is allowed to make a translational movement with 2 degrees of freedom with respect to a bed 1 along an X-axis and a Z-axis, which are translational axes orthogonal with each other. A table 3 is allowed to make a rotational movement with 1 degree of freedom with respect to a cradle 4 along a C-axis which is a rotational axis, and the cradle 4 is allowed to make a rotational movement with 1 degree of freedom with respect to a trunnion 5 along an A-axis which is a rotational axis. The A-axis and the C-axis are orthogonal with each other. Further, the trunnion 5 is allowed to make a translational movement with 1 degree of freedom with respect to the bed 1 along a Y-axis which is a translational axis orthogonal to the X-axis and the Z-axis. Accordingly, the spindle head 2 has 3 degrees of freedom of translational movement as well as 2 degrees of freedom of rotational movement, with respect to the table 3. Therefore, not only a relative position between the workpiece and the tool but a relative posture therebetween can also be controlled during processing.
As factors influencing geometric accuracy of movements of the five-axis machine, geometric errors between the axes (hereinafter simply referred to as geometric errors) are known, which include, for example, a center position error of the rotational axis (i.e., displacement from a supposed position) and a tilt error of the rotational axis (i.e., deviation in parallelism between the rotational axis and the translational axis). Although geometric errors such as deviation in perpendicularity of two translational axes relative to one another are present in the three-axis machine, the number of geometric errors is larger in the five-axis machine than in the three-axis machine because the number of axes is larger in the five-axis machine. To be more specific, a total of five geometric errors are possibly present in the three-axis machine, that is, three errors of perpendicularity between each of the translational axes and two errors of perpendicularity between the rotational axis of the spindle head and the translational axes. Meanwhile, in the case of the five-axis machine, for one rotational axis, the center position error of the rotational axis is possibly present in two directions and the tilt error of the rotational axis is also possibly present in two directions, so that four possible geometric errors are present per one rotational axis. Since the five-axis machine contains two rotational axes, eight geometric errors are possibly present. Further, as with the three-axis machine, five geometric errors are also possibly present in the five-axis machine in regard to the translational axes, i.e., errors of three perpendicularity between each of the translational axes, and two errors of perpendicularity between the rotational axis of the spindle head and the translational axes. Therefore, the total of thirteen geometric errors are possibly present in the five-axis machine.
Further, since the three-axis machine does not have any reference position such as the center of a rotational axis, a relative processing is carried out in the three-axis machine on the basis of an arbitrary processing point. On the contrary, an error occurs in the five-axis machine when a relation between a rotational axis and the workpiece or a relation between a rotational axis and the tool differs from a supposed relation. In other words, an influence of the geometric errors in the processing accuracy is more pronounced in the five-axis machine than in the three-axis machine. It can be said that, if the geometric errors are identified, highly accurate processing will be performed by means of various methods; for example, reducing the geometric errors by adjustment, controlling with a command program taking into consideration the geometric errors, and controlling for compensating the geometric errors. For this reason, it is extremely important for highly accurate processing in the five-axis machine to know the geometric errors.
As a first method for identifying geometric errors in the five-axis machine, Japanese Laid-open Patent Publication No. 2004-219132 proposes a method, in which a double ball bar measuring device is employed, which is capable of measuring a distance between centers of two balls, i.e., a spindle-side ball and a table-side ball, and one rotational axis is moved in synchronization with two translational axes which are moved along an arc in such a manner that the distance between centers of the spindle-side ball and the table-side ball is kept constant, and a relative displacement between the spindle-side ball and the table-side ball is measured. The obtained measurement data concern an arc trajectory, and some of geometric errors can be identified from the center deviation thereof. By changing the attachment direction of the ball bar, different geometric errors can be identified, so that the total of eight geometric errors in regard to the rotational axis can be identified.
As a second method for identifying geometric errors in the five-axis machine, Japanese Laid-open patent Publication No. 2005-61834 proposes a method, in which a ball is secured on the table and while indexing positions of the ball around a rotational axis, the center position of the ball is measured at plural different points using a touch trigger probe attached to the spindle head. A plane is calculated from the plurality of measured center positions of the ball, a vector normal to this plane is considered as an actual vector of the rotational axis, and a tilt error of the rotational axis is obtained from a difference between an ideal vector and the actual vector of the rotational axis. Further, the center position of the actual rotational axis is obtained from the plurality of measured values of the center positions of the ball in consideration of this actual vector, and a center position error of the rotational axis is obtained based on a difference between an ideal center position and the actual center position. Therefore, the total of eight geometric errors can be identified in regard to the rotational axis.
However, the first method employs a double ball bar, which is an expensive measuring device, and relatively skilled experience is required for manipulation of this double ball bar measuring device. Therefore, not everyone can easily implement this method.
In the second method, however, a touch trigger probe used in this method is relatively cheap and is usually provided in the machine tool as an option for the purpose of centering a workpiece, etc. Therefore, it is not necessary to additionally purchase a measuring device and the measuring device can be easily obtainable. Further, since the measurement operations are carried out by a control device, no particular skill is required and advantageously, measurements can be simply carried out. However, this method can only identify geometric errors in regard to the rotational axes, and disadvantageously geometric errors in regard to the translational axes cannot be identified. If geometric errors in regard to the translational axes are present, such errors influence the measured values of the center positions of the ball, which makes it impossible to accurately identify the geometric errors in regard to the rotational axes. Geometric errors in regard to the translational axes can be measured in advance using another method. However, since geometric errors vary in accordance with thermal displacement, secular change and the like, it may be difficult to accurately identify the geometric errors because of the influence of these changes. In order to know the geometric errors at a specific point of time, it is necessary to identify at the same time all the geometric errors.
In view of the above drawbacks of the conventional identification methods, the present invention seeks to provide a method and a program for identifying, substantially at the same time, geometric errors in regard to the translational axes in addition to geometric errors in regard to the rotational axes.